These days, most electric products employ a semiconductor device that includes a fine conductive circuit formed on a silicone chip. Generally, the semiconductor device is fabricated using a monocrystal silicon ingot as a starting material in accordance with the following procedure.
First, the ingot is sliced into thin pieces. The pieces are then polished in a lapping step and a polishing step to obtain bare wafers. The bare wafers include mirror surfaces and are thus referred to as mirror wafers. Also, if the bare wafers are obtained in an epitaxial growth layer forming step after the lapping step and before the polishing step, the bare wafers are particularly referred to as epitaxial wafers.
In a subsequent wafer treating step, the bare wafers are repeatedly subjected to oxidation, etching, and impurity diffusion. Afterwards, the bare wafers are cut into an appropriate size in a dicing step. This finally completes a desired semiconductor device.
In these steps, a device forming side of each semiconductor wafer needs be polished with a certain means. As an effective polishing means, various types of wafer polishing apparatuses (including lapping machines and polishing machines) have been proposed.
A typical wafer polishing apparatus includes a table, a pusher plate, and a cooling jacket. The table is secured to an upper portion of the cooling jacket. The table and the cooling jacket are formed of metal such as stainless steel. A passage is formed in the cooling jacket and coolant water for cooling the table circulates in the passage. The pusher plate is located above the table and has a holding side (a lower side) to which a wafer subject to polishing is adhered by a thermoplastic wax. The pusher plate rotates to press the wafer, which is held by the pusher plate, against a polishing side (an upper side) of the table from above. The wafer thus contacts the polishing side, and one side of the wafer is uniformly polished. During polishing, heat is generated on the wafer and is transmitted to the cooling jacket through the table. The coolant water that circulates in the passage of the cooling jacket releases the heat from the apparatus.
The table of the wafer polishing apparatus is often heated to a high temperature when polishing is performed. It is thus required that the table be formed of a heat-resistant and thermal-shock-resistant materials. Further, frictional force constantly acts on the polishing side of the table. It is thus required that the material of the table need be resistant to abrasive wear. In addition, generation of thermal stress that bends the wafer must be avoided to increase the wafer diameter and improve the wafer quality. It is thus necessary to minimize temperature differences in the table. Accordingly, the material of the table needs to have high heat conductivity.